1. Field of the Invention
Gas turbines for the highest requirements. Increasing the efficiency requires higher gas temperatures and hence higher temperature materials, more appropriate material combinations and better designs for the individual components. The most important and most critical of the components is the turbine blade.
The invention concerns the further development of mechanically and/or thermally highly loaded gas turbine blades, it being necessary to combine the advantageous properties of dispersion-hardened alloys for certain types of loading with those of non-dispersion-hardened alloys in an optimum manner.
In particular, it concerns a method for manufacturing a composite gas turbine blade consisting of root, airfoil and shroud plate or shroud, the airfoil consisting of an oxide-dispersion-hardened nickel-based superalloy in the condition of longitudinally directed coarse columnar crystals.
It also concerns a composite gas turbine blade consisting of a root, an airfoil and a shroud plate or a shroud, the airfoil consisting of an oxide-dispersion-hardened nickel-based superalloy in the condition of longitudinally directed coarse columnar crystals.
2. Discussion of Background
In rotating thermal machines (steam and gas turbines, for example), the ends of the blades are provided with shroud plates and/or shrouds at least in certain stages. The reasons for this are of a fluid mechanics, thermal and geometrical nature. These measures are therefore intended to improve the aerodynamics, the thermodynamics and the mechanics of the machine and to permit them to be designed with greater safety. in this connection, innumerable designs and material combinations are known for shroud plates and shrouds, their manufacture and the arrangements for fastening them to the tip end of the blade - including monolithic designs, forming an integral component with the airfoil. On this point, the following are some of the references which can be cited:
Walter Traupel, Thermische Turbomaschinen [Thermal Turbo Machines], Vol. 2, Regelverhalten, Festigkeit und dynamische Probleme [Regulating Behavior, Strength and Dynamic Problems], Springer Verlag 1960 PA1 H. Petermann, Konstruktion und Bandelemente von Stroemungsmaschinen, Springer Verlag 1960 Fritz Dietzel, Dampfturbinen [Steam Turbines], Georg Liebermann Verlag 1950 PA1 Fritz Dietzel, Dampfturbinen, Berechnung, Konstruktion [Steam Turbines, Calculation and Design], Carl Hauser Verlag.
Oxide-dispersion-hardened nickel-based superalloys have recently been proposed as the blading materials for highly loaded gas turbines because they permit higher operating temperatures than conventional cast and forged superalloys. In order to obtain the best strength values (high creep strength) at high temeratures, components in these alloys are employed with coarse crystallites longitudinally extended and directed along the blade axis. In the course of manufacture, the workpiece (semi-finished product or blank) generally has to be subjected to a zone heat treatment process. For various reasons (thermodynamics, crystallization laws), there are limits to the cross-sectional dimensions of such blading materials in the coarse-grained condition. In consequence, limits are also set to the blading dimensions. Now since the area of a shroud plate is generally several times the cross-sectional area of the corresponding blade airfoil, it is no longer possible, beyond certain dimensions, to manufacture blade and shroud plate monolithically from one piece. The same applies to the root part of the blade, which can become very voluminous in relative dimensions. If oxide-dispersion-hardened superalloys are to be successfully and generally employed, there is therefore a requirement for a division between the blade airfoil on the one hand and the shroud plate and root on the other. There are other reasons for such a division because of the strength and the material load at the clamping positions. Although a purely mechanical fastening of the shroud plate at the tip end of the blade airfoil can, fundamentally, solve the problem, it is expensive, requires additional fastening elements and can lead to additional operational stresses which are difficult to control. A welded connection is excluded because the structure of the oxide-dispersion-hardened material is substantially destroyed by the local melting. A connection by means of brazing or diffusion bonding demands very carefully machined contact surfaces and is associated with technological difficulties.
Casting in metallic workpiece parts, and casting around them, using a metallic material --usually of a lower melting point --is, in itself, known state of the art from numerous applications. It has already been proposed, inter alia, to cast steel into cast iron. It is then necessary to ensure that the steel has, as far as possible, a thermal expansion coefficient which is smaller or at most equal to that of the cast iron. Suitable steels for this purpose are, for example, those with 10 to 18% chromium content. The method has been used, inter alia, for casting around turbine blades (cf. CH-A-480 445). Intermediate layers of oxides are then said to be advantageous.
In the construction of highly loaded thermal machines, in particular gas turbines, there is a large requirement to employ oxide-dispersion-hardened superalloys to an increased extent and, consequently, to provide the designer with the technological means permitting him to employ these alloys in a substantially optimum manner while retaining the maximum possible design freedom.